In refiners designed for the manufacture of paper pulp, the size of the grinding gap between a stator and a rotor changes during operation as a result of wear on the grinding segments on the stator and rotor that face each other. For reasons of quality, it is desirable to have good control over the size of the grinding gap and to be able to change the size of the grinding gap in order, for example, to be able to compensate for wear. Refiners of this type have normally long operating periods, often several months, for which reason monitoring of the size of the grinding gap should be able to be carried out during operation.
In order to measure the size of the grinding gap, it is customary to use sensors of the magnetic type that are positioned stationary in the stator, with the end surface of the measuring head on a level with the surface of the grinding segments. For sensor calibration, the rotor is moved first axially in the direction towards the stator, while rotating and during idle running, until the grinding segments of both stator and rotor come into contact with each other. In this state, the sensor is zeroed. By then moving the rotor back a predetermined distance, the sensor can be calibrated. One of the disadvantages of such a method is that it can only be used on machines where the rotor can be operated with precision, as otherwise the wear caused by the zero-setting can be considerable and, in addition, the calibration and monitoring of the sensor can only be carried out during idle running.
A better method has proved to be to mount a sensor of the magnetic type in the stator in such a way that it can be moved, in order to be able to bring the sensor into contact with the rotor during calibration without needing to move the rotor axially. By this means, the change in position of the sensor is recorded close to the end of the sensor facing away from the grinding segment, thus at a large distance from the end surface of the measuring head. This results in a relatively large distance between the measurement position and the end surface of the sensor, with consequent difficulties in ensuring the required precision in the measurements, as during operation the sensor is often subjected to a change in length due to temperature variations. Against this background, there is a need for improved solutions within this field.